A power amplifier (PA) operates most efficiently when matched to its load. In most power amplifiers, and especially in the high-efficiency types, the tuning network is an integral part of the amplifier and proper tuning is essential for proper operation. Proper tuning promotes not only high efficiency, but also output power, stability, gain, and other desirable operating characteristics.
The tuning of power amplifiers has to date been accomplished by either fixed, mechanically tuned, or switched components. Fixed-tuned amplifiers have the obvious limitation of allowing proper operation over only one small range of frequencies or load impedances.
Mechanically variable tuning components include moveable-plate capacitors and roller inductors. While these extend the range of frequencies and impedances over which a PA can operate, changing frequency is a relatively slow process that must be done manually or by motors. This makes it too slow and cumbersome for many applications such as frequency-hopping and chirp signals.
Switching of tuning components can be accomplished by relays, pin diodes, MEMs (micro-electromechanical systems), MOSFETs, MESFETs, and other semiconductors. Typically, component values are selected in binary steps (1, 2, 4, 8, etc.) and a subset of the components is selected to approximate the desired capacitance or inductance. Relays offer low insertion loss, but are relatively large and slow. The various semiconductor switches offer high speed, but can have higher insertion loss. Additional loss occurs because of the components (e.g., RF chokes) that are required to feed the control signals to the switching components. There is an inherent trade-off between the range and accuracy of tuning and the number of components needed. Tuning over a large range with high accuracy can require an impractically large number of components.
Amplitude-modulated signals have to date been produced primarily by linear amplification or high-level amplitude modulation. Linear amplification offers wide bandwidth but is inherently inefficient. High-level amplitude modulation offers efficiency, but its bandwidth is limited by that of the high-level modulator.
The amplitude of the output of a power amplifier can be controlled by varying its the components in its tuning network. Mechanically tuned components cannot be varied fast enough to induce amplitude modulation at useable bandwidths. Switched components can in some cases adjust the tuning network fast enough, but the stepped nature of the resulting amplitude variation is unsuitable for high-quality amplitude-modulated signals. Variable attenuators can induce amplitude modulation, but do so by dissipating a significant portion of the power, resulting in an inefficient transmitter. Thus no currently existing techniques are capable of high-level amplitude modulation with significant bandwidth, quality, and/or efficiency.
Electronic tuning of small-signal circuits is a well-known art and can be accomplished by a variety of techniques. Varactor diodes are commonly used in applications such as voltage-controlled oscillators, phase shifters, frequency modulators, and phase modulators. Recently, micro-electromechanical systems (MEMS) and ceramic (especially barium-strontium-titanate, BST) devices have been developed for similar purposes.
Changes in inductance of ferromagnetic material with dc bias can also be used for electronic tuning. Because of nonlinearities, such tunable inductors (transductors) are used primarily in small-signal circuits such as receivers or low-power oscillators.
Electronically tuned filters for small-signal applications can also be implemented using an active-circuit “gyrator” to simulate the inductors. Such circuits are, however, unsuitable for use with power amplifiers as more power is required to operate them than is saved by the tuning process.
Variable ferrite inductors and transmission lines have been used in matching networks for plasmas. In such applications, the production of a clean, harmonic-free signal is not required, nor is modulation of the output signal.
The amplification of amplitude-modulated signals (including complex signals) has traditionally been done by linear power amplifiers (PAs). Since the efficiency of linear PAs varies with signal amplitude, such PAs are very inefficient for production of signals with significant peak-to-average ratios. The Kahn envelope-elimination-and-restoration technique ideally offers high efficiency at all signal amplitudes and has recently demonstrated significant improvements in average efficiency for amplitude-modulated signals. However, its bandwidth is limited by that of its high-level amplitude modulator.